California
Toxic algae in Mariposa County lakes
8 lakes and waterways in Mariposa County have harmful algal bloom reports in California's official data. The most recent entry is for Merced River: There are no bloom reports for Merced River in the last 30 days of California's data. The most recent was an algal mat alert sign on 30 August 2022 at El Portal. Merced River has had blooms in three or more years.
Lakes with bloom history
Newest report first. Each lake has its own page with the full history and what the results mean.
Other waterbodies with a report
These have one or two reports in a single year, which isn't enough history for a page of their own yet.
- East Fork Chowchilla River 2023
- Lake McClure 2024
- Lake McSwain 2026
- North Crane Creek 2025
- South Fork Merced River 2022
- Tenaya Creek 2021
- Yosemite Creek 2025
Source: California State Water Resources Control Board, official data, checked 24 October 2026. For today's conditions see the California HABs Portal map. No report doesn't mean a lake is safe: most water is only tested occasionally.